1. Field of the Invention
The present invention relates to an ultrasonic receiving apparatus for use with ultrasonic diagnosis systems and, more particularly, to an ultrasonic receiving apparatus suitable for high quality picture acquisition and having a low-cost digital reception beam former.
2. Description of the Related Art
Ultrasonic diagnosis systems have been in use for some years. These systems diagnose diseases of the internal organs in the human body by emitting ultrasonic waves thereto and by receiving what is reflected by the body's organ. Conventional ultrasonic diagnosis systems operate on the so-called dynamic focusing method. This method works as follows.
FIG. 16 is a schematic view of the relationship between a group of vibrators for emitting and receiving ultrasonic waves on the one hand, and ultrasonic reflecting points in the subject such as the human body on the other. FIG. 17 is a schematic view showing how the signals received by the vibrators are illustratively delayed for obtaining the information on points of ultrasonic reflection.
In both FIGS. 16 and 17, the horizontal direction is the direction in which n (e.g., 128) vibrators 1(1), 1(2), . . . , 1(n) are laid out in contact with the surface of the subject body. The vertical axis of FIG. 16 represents the depths d of ultrasonic waves as they are emitted into and reflected from the subject body. The vertical axis of FIG. 17 represents the delay times D of delay circuits (to be described later) connected to the vibrators 1(1), 1(2), . . . , 1(n). In this example, it is assumed that ultrasonic waves are reflected at points P1, P2 and P3 on a perpendicular extended from the center 0 of the vibrator group 1, as depicted in FIG. 16.
Where ultrasonic waves are reflected at point P1 having a depth of d1, the distance between point P1 and each of the vibrators 1(1), 1(2), . . . , 1(n) increases the farther the vibrator is from the center 0 of the vibrator group 1. When the ultrasonic waves reflected at point P1 reach the two vibrators 1(1) and 1(n) at opposite ends of the vibrator group 1, the waves will have reached the points constituting a circular arc A1 around its center point P1 (assuming that the medium is uniform for the waves). Therefore, to obtain the information about point P1 based on the received ultrasonic waves requires delaying the signals received by the vibrators 1(1), 1(2), . . . , 1(n) in the manner illustrated by a curve D1 in FIG. 17, before the signals are added up. Likewise, the ultrasonic waves are reflected at point P2 having a depth of d2 and at point P3 having a depth of d3. In this case, when the reflected waves reach the two vibrators 1(1) and 1(n) at opposite ends of the vibrator group 1, the waves will have reached the points constituting circular arcs A2 and A3 (also assuming that the medium is uniform for the waves). Thus the information about points P2 and P3 is obtained by delaying the signals received by the vibrators 1(1), 1(2), . . . , 1(n) in the manner depicted by curves D2 and D3 in FIG. 17, before the signals are added. The deeper the point at which the ultrasonic waves are reflected, the longer it takes for the reflected waves to reach the vibrators 1(1), 1(2), . . . , 1(n). Given this characteristic, the delay times D of the signals received by these vibrators are varied illustratively from D1 to D2 to D3 before the signals are added. This causes the ultrasonic focal point to move successively from point P1 to point P2 to point P3 inside the subject body. It is in the above manner that the dynamic focusing method is implemented.
FIG. 18 is a circuit block diagram of a conventional ultrasonic diagnosis system that operates on the dynamic focusing method using a digital circuit arrangement. In FIG. 18, a clock generation circuit 2 generates a clock signal CK having a predetermined cycle T and supplies it to a control circuit 3. The other component circuits operate in synchronism with the clock signal CK under control of the control circuit 3.
The vibrators 1(1), 1(2), . . . , 1(n) constituting the vibrator group 1 are connected with emission drivers 4(1), 4(2), . . . , 4(n) forming an emission driver group 4. During ultrasonic emission, the control circuit 3 sends timing pulses to the emission drivers 4(1), 4(2), . . . , 4(n). In turn, the emission drivers convert the timing pulses they received into high-voltage pulses and supply them to the vibrators 1(1), 1(2), . . . , 1(n). The high-voltage pulses cause the vibrators to emit pulse type ultrasonic waves into the subject body, not shown.
The vibrators 1(1), 1(2), . . . , 1(n) are also connected with A/D (analog to digital) converters 5(1), 5(2), . . . , 5(n) constituting an A/D converter group 5. Analog reception signals SA1, SA2, . . . , SAn received by the vibrators 1(1), 1(2), . . . , 1(n) are converted from analog to digital format by the A/D converters 5(1), 5(2), . . . , 5(n). After A/D conversion, the signals turn into digital reception signals SD1, SD2, . . . , SDn each comprising d bits (e.g., 8 bits).
The A/D converters 5(.1), 5(2), . . . , 5(n) are connected to delay circuits 6(1), 6(2), . . . , 6(n) constituting a delay circuit group 6. The digital reception signals SD1, SD2, . . . , SDn output by the A/D converters 5(1), 5(2), . . . , 5(n) are input to the respective delay circuits 6(1), 6(2), . . . , 6(n) for delay operations of predetermined times. The delay times of these delay circuits are controlled by delay control signals SC1, SC2, . . . , SCn transmitted from the control circuit 3. The delay circuits 6(1), 6(2), . . . , 6(n), output respective delayed digital reception signals SDD1, SDD2, . . . , SDDn which in turn are input to an adder 7. The add operation by the adder 7 produces an image display signal SI.
The conventional ultrasonic diagnosis system of the above construction produces the image display signal SI when controlled as follows.
During ultrasonic emission, ultrasonic waves are illustratively focused first on point P2 in the subject body as shown in FIG. 16. The focusing is achieved by the control circuit 3 sending appropriate timing pulses to the emission drivers 4(1), 4(2), . . . , 4(n). The timing pulses cause the emission drivers to activate the respective vibrators 1(1), 1(2), . . . , 1(n) with the timings progressively delayed the farther the vibrator is from the center 0 of the vibrator group 1 in accordance with the delay times D indicated by the curve D2 in FIG. 17.
The emitted ultrasonic beam is reflected inside the subject body at points where acoustic impedance is unmatched, and returns to the vibrator group 1. The reflected ultrasonic waves are received by the individual vibrators 1(1), 1(2), . . . , 1(n). The received waves are converted by the A/D converters 5(1), 5(2), . . . , 5(n) into the digital reception signals SD1, SD2, . . . , SDn for input to the delay circuits 6(1), 6(2), . . . , 6(n). These delay circuits output the delayed digital signals SDD1, SDD2, . . . , SDDn. The delay times are chronologically varied as shown in FIG. 17, from the delay pattern D1 to D2 to D3, so that the focal point of ultrasonic reception shifts from point P1 to P2 to P3, as depicted in FIG. 16. After being output, the delayed digital signals SDD1, SDD2, . . . , SDDn are added by the adder 7. In this manner, the dynamic focusing is implemented in a high resolution ultrasonic diagnosis system.
FIG. 19 is a block diagram of the delay circuit 6(1), one of the circuits constituting the delay circuit group 6 of FIG. 18. The same construction applies to the other delay circuits 6(2), 6(3), . . . , 6(n).
The digital reception signal SD1 made of d (e.g., bits (S0, S1, . . . , Sd) is generated by the A/D converter 5(1) of FIG. 18. This signal is input to the first 8(1) of m shift registers of d bits long each, the registers constituting the delay circuit 6(1). The shift registers 8 are driven by the clock signal CK generated by the clock generation circuit 2 of FIG. 18. Every time a cycle T of the clock signal CK elapses, the digital reception signal SD1 is shifted to the next shift register 8. The outputs of the shift registers 8(1), 8(2), . . . , 8(m) are connected respectively to tri-state buffers 10(1), 10(2), . . . , 10(n) of d bits each, the buffers forming a tri-state buffer group 10. In turn, the outputs of the tri-state buffers 10(1), 10(2), . . . , 10(n) are interconnected before they are connected to the adder 7 of FIG. 18. Each of the tri-state buffers 10(1), 10(2), . . . 10(n) operates in one of two modes. In the first mode, a d-bit signal whose pattern is made of 1's and 0's input to the buffer is output therefrom unchanged. In the second mode, the output of the buffer is held at the high impedance level. A decoder 9 is connected to the tri-state buffers 10(1), 10(2), . . . , 10(n). This decoder 9 decodes the delay control signal SC1 coming from the control circuit 3 of FIG. 18. By so doing, the decoder 9 places one of the m tri-state buffers 10(1), 10(2), . . . , 10(n) in the first mode and puts the remaining tri-state buffers in the second mode. In this manner, the delay circuit 6(1) outputs to the adder 7 a digital reception signal SDD1 delayed by an integer multiple of the cycle T, the delay cycle count being determined by which of the tri-state buffers 10(1), 10(2), . . . , 10(n) is placed in the first mode. In each of the delay circuits 6(1), 6(2), . . . , 6(n) making up the delay circuit group 6 of FIG. 18, each of the digital reception signals SD1, SD2, . . . , SDn input to these circuits is delayed so as to implement the above-mentioned dynamic focusing method. The digital reception signals SDD1, SDD2, . . . , SDDn coming from the delay circuits 6(1), 6(2), . . . , 6(n) are added by the adder 7 as described. This results in the output of the signal SI corresponding to a single scanning line on a monitor screen. When the above signal receiving process is repeated while the ultrasonic beam is being emitted successively in different directions, a two-dimensional tomographic view is formed on the monitor screen.
To meet the requirements of lower costs and higher quality picture acquisition of recent years, the ultrasonic diagnosis system that performs the above-described digital processing must address two problems: the problem of how accurate delay times should be in implementing the dynamic focusing, and the problem of how to construct a viable adder for adding numerous digital reception signals.
The problem of delay time accuracy will now be described. FIG. 20 is a schematic view of ultrasonic reception signals. The analog signals SA1, A2, . . . , SAn are obtained by the vibrators 1(1), 1(2), . . . 1(n) (see FIG. 18) which receive ultrasonic waves reflected from inside the subject body. These analog reception signals are not single pulse signals but substantially sine wave signals with a center frequency of M (e.g., 3.5 MHz) and having pulse type envelopes, as shown in FIG. 20. In FIG. 20, the signal (a) is a signal received by the centrally located vibrator of t e vibrator group 1, and the signal (b) is received by the vibrator located at either end of the vibrator group. Small circles in the waveforms represent points sampled at intervals of cycle T. In this setup, the delay circuit group 6 of FIG. 18 delays by an integer multiple of cycle T (e.g., 4 cycles) the signal (a) received by the centrally located vibrator, in order to generate the signal (c). The signals (b) and (c) are then added up.
At this point, adding the signals of FIG. 20 requires matching closely their envelopes EN1 and EN2 and synchronizing the respective phase of the more or less sine wave signals SS1 and SS2. If the signals are added out of phase, the result of the addition is not meaningful. Experiments and experience have shown that the phase accuracy in that case should be within .+-.22.5.degree.. This means that to attain the necessary time delays using the delay circuits 6(1), 6(2), . . . , 6(n) of FIG. 18 with respect to the above degree of accuracy requires having the A/D converters 5(1), 5(2), . . . , 5(n) perform sampling at intervals of a cycle one eighth or less of one M-th or less of the center frequency M. That is, the repeat frequency f of the clock signal CK should be at least eight times the center frequency M of the signals SS1 and SS2. Today, the ever-increasing needs for higher quality pictures are being met by ultrasonic diagnosis systems using higher ultrasonic frequencies. Some systems utilize ultrasonic probes of as high as 5 MHz and 7.5 MHz in frequency. The use of the 5 MHz and 7.5 MHz probes with ultrasonic diagnosis systems calls for high frequency clock signals CK of 40 MHz and 60 MHz, respectively. The received signals have each a dynamic range of at least 40 dB, the range varying somewhat depending on the intensity of reflectance at various points of reflection in the subject body. Thus to obtain a tomogram requires the use of a signal of at least 8 bits per point of reflection. The above process involves employing high-speed high-resolution A/D converters and high-speed circuits for adding digitized reception signals for delay. Because of their scale and speed requirements, these circuits are too costly to gain widespread acceptance in ultrasonic diagnosis systems.
One prior art solution to the above difficulty is as follows. Sampling is performed by use of a clock signal CK having a repeat frequency about double the highest frequency on the reception signal band (e.g., 7.5 MHz to 10 MHz for the center frequency of 5 MHz). Points between the sampled points are interpolated from the signal corresponding to each sampled point. The signals obtained by interpolation are also regarded as reception signals. The signals are then added up so that the phase matching accuracy between signals will fall within the range of .+-.22.5.degree. for delay. One disadvantage of this solution is that the use of primary interpolation calculations aimed at high-speed processing entails too low levels of interpolation accuracy, while secondary or higher degrees of interpolation calculations if adopted involve complex computations that require high-speed processing. These requirements call for a vastly enlarged scale of circuitry.
Another prior art solution to the above difficulty is proposed in Japanese Patent Laid-Open No. 1-31151. This solution involves first using a clock signal CK about double the highest frequency on the reception signal band. Each of the vibrators 1(1), 1(2), . . . , 1(n) is connected with two A/D converters. The two A/D converters are shifted from each other in sampling timing by a phase difference of 90 degrees relative to the center frequency of the reception signal. One disadvantage of this solution is that each vibrator needs two A/D converters, two delay circuits and two adders, in addition to such arithmetic circuits as multipliers and switchers. That is, the scale of the circuits involved will have to be at least doubled.
Another difficulty with conventional ultrasonic diagnosis systems is the ever-increasing complexity of the adder in its circuit construction. In recent years, the need for higher quality pictures has necessitated not only the adoption of high frequency ultrasonic waves but also the use of larger aperture of ultrasonic probes. In some systems, the number of vibrators can be as large as 128. In that case, a logic circuit arrangement for adding 128 digital reception signals all at once would be very complicated indeed. Such logic circuits are difficult to implement because of their scale and of the long operation times involved.